Exercise 1.8
(define (cubic-iter guess previous_guess x)
  (if (good-enough? guess previous_guess)
      guess
      (cubic-iter (improve guess x) guess
                 x)))

(define (improve guess x)
  (/ (+ (/ x (* guess guess)) (* 2 guess)) 3))

(define (good-enough? guess previous_guess)
  (< (abs (- previous_guess guess)) (abs (* 0.0000000000001 guess))))

(define (cubic_root x)
  (cubic-iter 1.0 2.0 x))

